Thermal protection method for electroanalytical cell and amperometric probe

ABSTRACT

A method of thermal protection of a membrane-enclosed amperometric cell of the type comprising an electrolyte within an electrolyte space containing electrodes and enclosed by a semi-permeable membrane which defines a sensor face; heat exchange within said cell is effected for controlled heat compensation of the electrolyte when the sensor face is exposed to temperatures outside of the operative temperature range of the cell. 
     A membrane-enclosed amperometric probe having an elongated body provided with a sensor face at one end of the body and with electrical electrode connections extending through the probe body; the body includes at least two elongated fluid-passing conduits which are not in fluid communication with the electrolyte space and which extend in axial direction through the body from a first region near the one body end to a second region near the other body end; the conduits have mutually communicating ends in the first region and separate fluid-feeding ends in the second region so as to permit passing a fluid through the probe body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of our commonly assigned,copending U.S. patent application Ser. No. 06/345,536, filed Feb. 3,1982, now U.S. Pat. No. 4,518,477, granted May 21, 1985.

This application generally relates to subject matter disclosed in ourcommonly assigned U.S. application Ser. No. 773,163 filed Mar. 1, 1977,issued as U.S. Pat. No. 4,096,047 on June 20, 1978, as well as tosubject matter disclosed in our commonly assigned U.S. application Ser.No. 164,291, filed June 30, 1980, now U.S. Pat. No. 4,325,797, grantedApr. 20, 1982, and the copending divisional application Ser. No.06/319,708, filed Nov. 9, 1981, now U.S. Pat. No. 4,372,021, grantedFeb. 8, 1983.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

This invention relates generally to the art of amperometric measurementand amperometric measuring devices of the type used for quantitativeelectrochemical analysis methods where the concentration of anelectroactive species such as oxygen dissolved in or admixed with afluid is to be measured or monitored; more particularly, this inventionrelates to an improved method of thermally protecting an amperometriccell or probe which is temporarily subjected to temperatures outside ofthe operative cell temperature range, such as in heat sterilization.

The invention further comprises an improved probe provided for passageof a fluid, such as a cooling liquid, through said probe.

(b) Description of the Prior Art

Electrochemical cells of the type used for quantitative electrochemicalanalysis are well known in the art and generally include a working orsensing electrode having a defined or electroanalytically effectivesurface, a counter electrode, an optional guard electrode, anelectrolyte in contact with the electrodes and a membrane that issubstantially impermeable to the electrolyte but is permeable to theelectroactive species of interest and defines the sensor face in termsof a barrier between the electrolyte space, notably the electrolyte filmon top of the sensing electrode, and the ambient medium that containsthe electroactive species.

For amperometric analytical operation, the working electrode of such acell arrangement is polarized by a constant DC voltage to furnish acurrent whose steady state magnitude is proportional to the activity ofthe electroactive species of interest. Cells of this type and theiroperation and uses are discussed in detail in the following illustrativeU.S. Pat. Nos. 2,913,386, 3,071,530, 3,223,608, 3,227,643, 3,372,103,3,406,109, 3,429,796, 3,515,658, 3,622,488 and 4,096,047 as well as inBritish Published Application No. 2,013,895.

Structural and operational data of such prior art cells can be found inthe literature, particularly in the Monography by Fatt, Irving,"Polarographic Oxygen Sensors", CRC-Press, Inc., USA, 1976, incorporatedherein by reference.

The first mentioned U.S. Pat. No. 2,913,386 to Leland E. Clarkconsidered as the pioneering patent in this art already teaches asemi-permeable membrane that restrains the electrolyte and the terms"membrane-covered" or "membrane-enclosed" are being used generally torefer to such electroanalytical devices, e.g. as "membrane-coveredpolarographic detectors".

As the term "polarography" has also been used for techniques based onthe dropping mercury electrode and operating either in a voltametric orgalvanic mode, the term "membrane-enclosed amperometric cell" or MEAC isused herein to refer to electroanalytical probes such as the "ClarkCell" and modifications thereof including those that use guardelectrodes and various devices to improve operation, reliability,sensitivity and maintenance.

Because of the high sensitivity, e.g. for routine determinations ofdissolved oxygen in water in the ppm to ppb range, MEAC-type oxygenprobes are of interest in various types of industrial microbiologicalprocesses including enzymatic or fermentative methods, where reliablemonitoring of the oxygen content is of paramount interest.

Microbiological methods require that the equipment which is exposed to abiologically active and, hence, biodegradable medium must besterilizable; heat sterilization, such as exposure to temperatures above100° C., e.g. pressurized steam of 120° C., for periods in the range ofminutes or hours is a generally preferred method for heat sterilizationof processing equipment, such as fermenters, feeding pipes and otherdevices that have come, or will come, in contact with a process streamcontaining biologically active matter.

As a consequence, oxygen probes for use in such processes should beheat-sterilizable but because MEACs include temperature sensitiveconstituents, one of which is the electrolyte, this presents substantialproblems. Thus, when a MEAC typically designed for operation at atemperature between 0° C. and 40° C. is exposed to 120° C. steam, thefollowing damaging effects may occur:

expansion of the volume of the e1ectrolyte relative to the spaceavailable, creating internal pressures which tend to stretch themembrane;

shrinkage, softening and creeping of structural parts of the probeproducing permanent dimensional changes;

stresses of differential expansion of disparate materials, leading topermanent damage of structural parts;

recrystallization or phase changes of the membrane material causingpermeability changes which, even if but temporary, can cause measurementerrors for a time;

enhanced solvent power and corrosiveness of electrolyte at elevatedtemperatures causing attack on structural parts.

A conventional way around these problems is to use a MEAC that can betaken out of the system to be monitored when the latter isheat-sterilized and to effect sterilization by other than thermaltreatment. As actual measurement is neither required nor normallypossible during heat-sterilization periods, this is feasible per se butrequires a substantial procedural effort, aside from the problems of areliable sterilizing treatment for the probe that does not introduceuncontrolled sterilant residues into the system which is to bemonitored.

Cooling of the MEAC would seem to provide another way to resolveheat-sterilization problems; yet, probe cooling implies that exposedparts of the probe do not reach sterilization conditions and remaincontaminated.

Finally, use of a non-aqueous electrolyte solvent having a boiling pointabove 120° C. and a low vapor pressure at that temperature would seem tooffer another way around some of the difficulties set forth above but asthe requirements for such a solvent are quite demanding, no practicablesolution appears feasible.

The operative temperature range of a MEAC containing an aqueouselectrolyte, i.e. the temperatures of an ambient medium which do notdamage the MEAC or modify its sensitivity, may be broadened somewhat byelectrolyte additives, such as thickeners, and such additives are infact being used in prior art heat-sterilizable sensors which, inaddition to electrolyte additives, provide for mechanically compensatingthe thermal expansion of the electrolyte during in-lineheat-sterilization. The disadvantages of such a system are decreasedsensitivity and less reliability, prolonged stabilization periods,frequent exchange of membrane and electrolyte, and relativelycomplicated structures and maintenance requirements.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is a main object of the invention to provide for amethod of thermally protecting a MEAC or probe having an operativetemperature range typical for aqueous electrolytes when the MEAC is tobe kept in-line during heat-sterilization periods so that the sensorface, at least, of the MEAC is exposed to temperatures substantiallyabove the operative range of the MEAC.

Another object of the invention is to provide for a MEAC-type probesuitable for in-line use in an environment that is subject to periodicheat-sterilization treatment.

A further object of the invention is to provide for a MEAC-type probethat permits passing of a fluid into and out of the cell structure.

Yet another object of the invention is to provide for a MEAC-type probehaving a shield against thermal shock.

Further objects will become apparent as the specification proceeds.

It has been found according to a first general method embodiment of theinvention that the above objects and further advantages will be achievedwhen effecting a heat exchange within a MEAC for controlled heatcompensation of the aqueous electrolyte within the MEAC, generally byheat exchange with the electrolyte, when exposed to temperaturessubstantially outside of the normal operative temperature range.

According to a first general apparatus embodiment the invention providesfor a MEAC probe that includes, in its body, at least two elongatedfluid-passing conduits which are not in fluid communication with anelectrolyte space and which extend in axial direction through said bodyfrom a first region near one end of the body to a second region near theother body end; the conduits have mutually communicating ends in thefirst region and separate fluid-feeding ends in the second region andpermit passing of a fluid through the probe body.

The heat exchange with the aqueous electrolyte is effected by controlledheat compensation and this includes operation in a manner that does notprevent sterilization of those external or "exposed" portions of theprobe that have, or are intended to, come into contact with biologicallyactive medium that is to be monitored.

Surprisingly, it has been found according to the invention thatcontrolled heat compensation of the electrolyte by direct or indirectheat exchange therewith provides for sterilizing conditions at theexposed probe portions including the sensor face without adverse effectson the MEAC when operating according to the invention. While not wishingto be bound to a specific theory, it may be assumed that the heatcapacity of the aqueous electrolyte and its relatively poor heatconductivity is the main reason that controlled electrolyte heatcompensation according to the invention is capable of resolving theproblems encountered with conventional sterilizable MEAC probesincluding those operating with expansion control.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferably, the heat exchange step according to the inventive methodincludes passing a stream of a heat transfer fluid, preferably a liquid,through the cell for indirect or direct heat exchange with theelectrolyte. In direct heat exchange, the electrolyte may be used as theheat transfer fluid; for economy and convenience of operation anindirect heat exchange within the MEAC is used for controlled heatcompensation of the electrolyte, and a heat transfer fluid, such aswater, is passed through the cell to contact a heat conductive cellmember, such as an electrode, which in turn contacts the electrolytespace and thus is in a heat conducting connection with the electrolytetherein.

It is preferred for many purposes of the invention to effect theindirect heat exchange with the electrolyte by passing the heat transferfluid through a substantially annular and confined fluid-passing channelor chamber sealed from the electrolyte space but in heat exchange orheat conducting contact therewith by a heat conducting cell member,preferably the outermost electrode of a coaxial electrode arrangement.

Such coaxial electrode arrangements include a central electrode, e.g.the sensing electrode or cathode, and at least one peripheral electrode,e.g. the counter electrode or anode, that extend around the centralelectrode. Optionally, a guard electrode may be included in the coaxialelectrode arrangement between the central electrode and the outermost orperipheral electrode, and an electrical insulator is provided betweenany two adjacent electrodes at least. Such coaxial arrangements areknown per se, i.e. without fluid circulation means, and are disclosed,for example, in our above-mentioned U.S. Pat. No. 4,096,047 and inBritish Published Application No. 2,013,895, both of which areincorporated herein by way of reference.

Preferred coaxial electrode arrangements with or without a guardelectrode include a well portion or electrolyte reservoir in theelectrolyte space, and the confined annular fluid-passing channel orchamber should be in heat exchange contact with such well portion.Passing of heat transfer f1uid through the annular channel or chamberpreferably is effected by fluid-passing conduits or conduit means whichare not in fluid communication with the electrolyte space and which maybe formed by grooves or the like recesses at the interface of a pair ofcoaxial tubular cell members explained in more detail below. Preferably,such a tubular pair is used to provide for a thermal shield that extendsthrough a major portion, at least, of the probe body within the probejacket, and a thermal insulator, such as an air gap, between jacket andshield can be provided to assure that the shield has no substantialcooling effect upon those outer jacket portions that are intended to beexposed to heat sterilization.

DEFINITION OF TERMS

The phrase "controlled heat compensation of the electrolyte" as usedherein refers to the condition of maintaining the electrolyte attemperature conditions that will not damage the cell or modify itssensitivity while the exposed external parts of the probe are subject totemperature conditions that without such heat compensation would damagethe cell, or modify its sensitivity. Such compensation is controlled bythe heat flux; for example, when heat-sterilizing (120° C.) a MEACaccording to the invention, a typical heat flux for controlled heatcompensation of the electrolyte when the MEAC is heat-sterilized may bein the order of 10 to 500 Kilojoules per hour and such heat flux, inturn, can be controlled by the passing rate of the heat transfer fluid,the temperature condition and type of heat transfer fluid used and otherparameters that are known per se in the heat exchange art. As anillustrative example when using tap water as heat transfer fluid in aprobe according to the invention, a rate of 2 to 20 liters per hour maybe sufficient for adequate heat flux control.

The term "operative temperature" of a MEAC is intended to refer to thattemperature range within which a given MEAC can be operated withoutadverse effects as to cell structure and sensitivity. It will beunderstood that maintaining the sensitive inner portions and notably theaqueous electrolyte of a MEAC at an operative temperature duringheat-sterilization does not imply actual operation (oxygen sensing) ofthe cell during heat-sterilization. In fact, while such continuedoperation is possible with the inventive method and apparatus,amperometric values measured in an ambient medium underheat-sterilization conditions with a probe in which the electrolyte isat a substantially lower temperature are not normally significant.

The terms "heat conductive" and "thermally conductive" are usedinterchangeably herein and refer to materials and connections having aheat conductivity, expressed in Joule.sec⁻¹.m⁻¹.K⁻¹, of above 0.5,preferably at least about 1.5; a preferred heat conductivity range isbetween 3.5 and 5.0 or more.

Correspondingly, the terms "thermally insulated" or "thermallyinsulating" refer to materials and connections having a heatconductivity of substantially below 0.5, e.g. below 0.05 or lower whenexpressed in terms of the above units.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which

FIG. 1A is a cross-sectional view illustrating the operation of theinventive method with reference to schematic example of an amperometricelectroanalytical cell including an internal heat exchange means andshown in a diagrammatic and fragmented presentation;

FIG. 1B is the top-view of the cell shown in FIG. 1A;

FIG. 2A is a cross-sectional diagrammatic view of a modification of thecell shown in FIGS. 1A, 1B;

FIG. 2B is a top-view of the cell of FIG. 2A;

FIG. 3 is a diagrammatic perspective view of a pair of tubular probemembers forming fluid-passing channels at their interface in themembrane-enclosed amperometric probe as shown in FIG. 5;

FIG. 4 is a diagrammatic perspective view of a modification of the pairof tubular probe members shown in FIG. 3;

FIG. 5 is a semi-diagrammatic sectional view of a member-enclosedamperometric probe according to the invention, and

FIG. 6 is a perspective view of the thermal shield constituent of theprobe of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A, 1B and 2A, 2B of the drawings illustrate a schematic exampleof an amperometric electroanalytical cell including heat exchange means;the example is shown merely for the purpose of explaining the workingsof the method according to the invention.

The diagrammatic sectional view of cell 10 shown in FIG. 1A includes aworking electrode or cathode 11, a counter electrode or anode 12, anelectrical insulator 15 between the electrodes 11, 12 and anelectrically insulating jacket 16 around electrode 12.

The electrolyte-containing space 13 is in contact with both electrodes11, 12 by means of an electrolyte well or reservoir 130 that contactsanode 12 and an electrolyte film 131 covering the surface of cathode 11.The electrolyte-containing space 13 is enclosed by membrane 14 held onjacket 16 by a membrane-holding member 18. Each electrode 11, 12 has aseparate electrical outlet or lead 115, 125 for connection with ameasuring circuit or instrument (not shown).

Membrane 14, e.g. a thin film of polytetrafluoro ethylene, isimpermeable to the electrolyte in space 13 but permeable to anelectroactive species in an ambient medium that is in contact with theouter surface of membrane 14.

The frontal sensor face on the elongated body of cell 10 is definedessentially by the outer surface of membrane 14 above electrolyte film131 on top of electrode 11 and the reaction of electroactive specieswill occur at the electrolyte-exposed surface of cathode 11. In order toprevent uncontrolled electrolyte contact of portions of cathode 11 atinterface 110 with insulator 15, a pressure seal of the type disclosedin our commonly assigned U.S. Pat. No. 4,096,047 is generally preferred.

In order to thermally protect cell 10 according to the inventive method,an annular confined channel or chamber 121 is provided and a heatconductive layer 120 separates chamber 121 from electrolyte space 13;layer 120 constitutes the electrolyte-exposed surface of the anode 12and consists of a metal suitable for use as anode, e.g. silver or thelike heat conductive and electrically conductive metal suitable for useas an electrode that can be "consumed" by the electrochemical reactionof interest. Preferably, layer 120 is an integral part of anode 12.

Two elongated conduits 124, 122 extend through the body of cell 10 sothat a heat transfer medium can be passed through annular chamber 121.

When the sensor face of cell 10 and generally an adjacent portion ofjacket 16 is exposed to heat-sterilizing temperatures of typically about120° C., the electrolyte near the sensor face would be heated throughmembrane 14 and reach temperatures far outside of the normal operativetemperature range of cell 10 with the consequence of damaging itsstructure or impairing its sensitivity.

According to the method of the invention, a heat exchange is effectedwithin cell 10 by passing heat transfer or coolant fluid, such as air orwater, into one conduit 122, 124 through chamber 121 and out through theother conduit; as a consequence, the electrolyte in electrolyte space 13adjacent to the heat conductive anode portion 120 will be in heatexchange contact with the heat transfer fluid passing through chamber121.

Heat compensation of the electrolyte in electrolyte space 13 can becontrolled simply by adjusting the rate of passage of the heat transferor coolant medium through chamber 121. Typically, with a convenientcoolant such as tap water having tap temperature in the range of fromabout 5° to 20° C., passage rates of typically between 0.5 and 2.5milliliters per second will be sufficient for satisfactory heatcompensation of the electrolyte during heat-sterilization at 120° C.

The difference between the temperature of the coolant at the inlet andoutlet ends of conduits 122, 124 can be used to ascertain that asufficient passage rate is maintained; typically, that difference shouldbe small, say below 5° C., and passage of coolant will be continued atleast as long as heat-sterilization proceeds. Sterilization periods ofminutes or hours are conventional in microbiological processes.

It is essential, according to the invention, that the heat exchangeexplained above should be effected within cell 10, i.e. without coolingany outer parts thereof, intended to be heat-sterilized, to temperaturesthat are too low for the sterilization effect.

It will be understood that the bottom end of cell 10 and an adjacentportion of jacket 16 are not intended for exposure to heat-sterilizationwhen cell 10 is maintained in-line in an apparatus; as is conventionalfor sterilizable probes, only its sensor face portion, optionally withan adjacent jacket portion, extending into the processing vessel orline, is in contact with biologically active medium therein; the bottomportion of the cell is maintained outside of the vessel and a seal,coupling or the like will be provided to maintain jacket 16 of cell 10in sealing engagement with the wall of the processing vessel or line. Assuch couplings and seals are conventional, this aspect will not bediscussed in more detail.

As will be apparent from the diagram of cell 10 in FIG. 1A, the passageof coolant through chamber 121 should not prevent the outer surface ofjacket 16 from reaching sterilizing temperatures where sterilization isrequired. Thus, a thermal insulation of the heat conducting member thatis in thermally conducting contact with the coolant, e.g. anode 12, andthe outer surface of jacket 16 may be required and is indicated,diagrammatically, in FIG. 1A in that jacket 16 is provided with arelatively thick wall 160 of a thermally insulating material as definedabove, e.g. an organic polymer.

However, additional heat insulation of the outermost heat conductivemember that is in direct thermal contact with the coolant (anode 12 inFIG. 1A) and an adjacent jacket (16 in FIG. 1A) may be desirable, and apreferred example will be explained below in connection with FIG. 5.

The top view of cell 10 in FIG. 1B shows the preferred coaxialarrangement of the electrodes 11, 12 and the preferred annular shape ofchamber 121 (broken lines) with the ends of conduits 122, 124 (brokenlines) in mutual communication through chamber 121.

Cell 20 shown diagrammatically in FIGS. 2A, 2B illustrates the preferredcoaxial electrode arrangement with an additional or guard electrode 23.Advantages and operation of guard electrodes are known per se, e.g. asdisclosed in our commonly assigned British Published Patent ApplicationNo. 2,013,895, and will not be explained herein.

In order to operate cell 20 of FIGS. 2A, 2B according to the presentinvention, the outermost or peripheral electrode, i.e. anode 22, of thecoaxial electrode structure (comprising cathode 21, guard electrode 23and anode 22 with electrical insulators 25, 27 between any two adjacentelectrodes) is provided with a confined annular channel or chamber 292separated from electrolyte space 299 by a top wall portion 220 of anode22. Again, portion 220 may be an integral part of anode 22 or affixedonto the latter but in either case consists of a heat conductive andelectrically conductive material, such as silver, to provide for thermaland electrical contact of anode 22 with the electrolyte in electrolytespace 299.

Two conduits 222, 224 extend from the lower end of cell 20 through thecell body and communicate, at their upper ends, through chamber 292.Each electrode is provided with a lead 215, 225, 235 for connection witha conventional measuring instrument (not shown).

For thermal protection of cell 20 when exposed at its sensor face (outersurface of membrane 24 on top of cathode 21) and, optionally, at anadjacent portion of jacket 26 to heat-sterilization, a coolant fluidwill be passed from the lower end of cell 20 (maintained outside ofvessel in which heat-sterilization is required as explained above) intoone of the conduits 222, 224, through annular channel or chamber 292 andout of the other conduit.

It will be noted that heat exchange between the conduits 222, 224 and anadjacent heat conductive tubular member, such as anode 22, of cell 20may serve to thermally protect the inner cell components (i.e. thoseencompassed by the heat conductive member) and this is advantageous asexplained below if thermal insulation, e.g. insulating wall 260, isprovided between that member and the outer surface portion of jacket 26that is intended to be heat-sterilized.

Again, the top-view shown in FIG. 2B is given to illustrate the coaxialelectrode arrangement and the position of annular channel or chamber 292in the outermost or peripheral electrode (anode 22).

It will be apparent from the explanation of FIGS. 1A, 2A that anindirect heat exchange is effected between the coolant in the separateannular channel or chamber 121, 292 and the electrolyte because of theintermediate heat conductivity of anode wall 120, 220. However, a directheat exchange is possible when operating the inventive method; in thatcase, anode wall portion 120, 220 is omitted and electrolyte iscirculated as coolant through the conduits but a separate heat exchangeris required for cooling the circulating electrolyte.

FIGS. 3 and 4 are intended to show, in a semi-diagrammatic perspectiveview, how the problem of passing a fluid through a MEAC-type probe issolved in a preferred embodiment of the membrane-enclosed amperometricprobe according to the invention described hereinbelow with reference toFIGS. 5 and 6.

Pairs 30, 40 of interfitting tubular members 31, 32; 41, 42 are used anda pair of axially extending channels 33, 34; 43, 44 is formed at theinterface of each pair 30, 40 by means of grooves or similar elongatedrecesses that may be formed in the inner surface of the outer tube (32in FIG. 3), or in the outer surface of the inner tube (41 in FIG. 4), orby a combination thereof (not shown).

For many purposes of the invention it is preferred that one tubularmember of the pair is made of a heat conductive material, e.g. a metalsuch as silver, copper or the like, or metal alloy, while the othermember is made of a thermally insulating material such as an organicplastic, e.g. selected from cured epoxides, polypropylene, Nylon 66,polyethylene, terephthalate, acrylics including polymethacrylic esters,polystyrene, polyvinyl chloride (unplasticized), high densitypolyethylene, polyvinylidene fluoride, polyvinyl carbazole, polyvinylacetate, polysulfones, polycarbonates including polybisphenol carbonate,polyphenylene oxide, polyurethane, polyacetals includingpolyoxymethylene and various copolymers including those made of styreneand acrylonitrile or of styrene, acrylonitrile and butadiene.

It will be understood that more than two channels can be provided at theinterface of the interfitting tubular members and that the grooves orother types of recesses or channel-forming distancing means could spiralaround the interface. Also, fluid-passing channels could be formedbetween different interfaces of interfitting tubular members as thepreferred pairwise arrangement of interfitting tubes for providingchannels at one interface is but a preferred embodiment. According to apreferred embodiment of the invention, such a pair of interfitting tubesis used as a thermal shield inside the outer jacket of a probe with athermally insulating layer, e.g. an air gap, between the probe jacketand the tube pair; in that case, it is preferred for many purposes thatthe outer member of the pair is made of a heat conducting material, seealso FIG. 6.

The channels 33, 34; 43, 44 of such pairs of interfitting tubes willhave, or be connected with, mutually communicating ends (not shown inFIGS. 3, 4) in a first cell or probe region near the probe sensor end,e.g. the annular chamber 577 of FIGS. 5, 6 or another type ofcommunicating passage which, in turn, preferably is in heat conductingconnection with the electrolyte space.

Further, these channels will have separate fluid-feeding ends (not shownin FIGS. 3, 4), generally with a connection or line for feeding heatexchange fluid, and these fluid-feeding ends will be in a cell or proberegion that is distanced from the sensor end and generally is near theother or opposite end of the cell or probe body. By this, it is meantthat the communicating ends are in that probe portion which at its outersurface is exposed to temperatures outside the operative range, e.g. toheat-sterilizing conditions, while the fluid-feeding ends of thechannels are in a region of the probe body that is not normally subject,at its outer side, to heat-sterilizing conditions. Generally, the "firstcell or probe region" is that part of the probe which will be situatedwithin a vessel, line, tank or other processing equipment that requiresoxygen monitoring and periodic heat-sterilization; by the same token,the "second region" of the cell or probe is that part of the probe whichextends outside of the walls of the processing equipment and is notnormally exposed to the heat-sterilization conditions within theequipment.

A preferred embodiment of a membrane-enclosed amperometric probeaccording to the invention is illustrated in a semi-diagrammaticalcross-sectional view in FIG. 5 showing a cell or probe in the form of acylindrical body 5 having a first probe portion 51 near sensor face 511and a second probe portion 52 near the opposite probe end 521 whereelectrical connections or leads 531, 541, 551 are provided forseparately connecting the central sensing electrode or cathode 53, theoptional intermediate or guard electrode 54 and the peripheral counterelectrode or anode 55 with a conventional amperometric measuring device(not shown).

For in-line operation of probe 5 according to the invention, the firstprobe portion or exposed end 51 of probe 5 will extend into a fermenter,feed-line, tank or the like enclosed equipment (not shown) formicrobiological, enzymatic or the like processing requiring periodicalheat-sterilization.

For operation, a pressure sealing means (not shown) will be connectedwith a portion of socket 592 or jacket 59 to ensure sealing connectionof probe 5 with an opening of the processing equipment and the secondprobe portion or external region 52 of probe 5. The first or immersedprobe portion 51 includes the electrolyte space 56 having a well orreservoir portion 561 and contacting the sensing electrode 53 by a thinfilm (not shown) of the electrolyte between the surface of electrode 53and the semipermeable membrane (not shown). An insulator 532 is inpressure engagement as disclosed in U.S. Pat. No. 4,096,047 with theelectrode 53 by means of a spring 533 between nut 534 on the threadedend of cathode-holding rod 535 and the end 542 of tube 545.

Metal tube 545 provides the electrical connection between guardelectrode 54 and its connecting lead 541. Metal tube 555 provides forelectrical connection between anode 55 and anode lead 551. The metaltubes 545, 555 extend through the cylindrical body 5.

A pair of coaxially arranged interfitting tubular members 571, 572around anode tube 555 forms a pair of elongated fluid-passing channels578, 579 by a pair of grooves as explained with reference to FIGS. 3, 4above and further shown in FIG. 6. Tube 571 is made of an organicpolymer, while tube 572 is made of metal. As evident from FIGS. 5 and 6,the fluid-passing channels 578, 579 axially extend through thecylindrical body 5 from the first region of the first probe portion 51into the second region of the second probe portion 52 and communicate attheir ends located in the first region and are connected tofluid-feeding ends located at the second region.

An annular channel or chamber 577 serves to connect the "upper" ends ofchannels 578, 579 while each lower end of channels 578, 579 is connectedwith a separate socket bore 593, 594 for feeding a heat transfer fluidinto one bore 593, 594 through one channel 578, 579 to the annularchannel chamber 577 and out through the other channel 579, 578 and theother bore 594, 593.

The inner wall portion of annular channel or chamber 577 is formed by aportion of anode 55 which consists of a metal, such as silver, which isheat conducting. As anode 55 also contacts electrolyte space 56,controlled heat exchange of the electrolyte in space 56 with the heattransfer fluid passing through annular channel 577 via the heatconducting material of anode 55 can be effected by controlling thepassage of heat transfer fluid.

As indicated, one of the interfitting tubular members 571, 572 formingchannels 578, 579 at their interface preferably is made of a heatconducting metal, e.g. the outer tubular member 572 in FIG. 5, while theother may be made of an organic polymer.

Because a coolant that passes through channels 578, 579 will have acooling effect on tube 572, a heat insulation between tube 572 andjacket 59 is provided by means of an annular air gap 581.

The upper end of metal tube 572 bears on jacket head 591 which consistsof a material having a low thermal conductivity, such as an organicpolymer of the type exemplified above. Jacket 59 (and an optional coaton jacket head 591) consists of a metal, such as stainless steel, butmight also consist of an organic polymer of the type illustrated aboveoptionally provided with a metal coating depending upon use requirementsincluding ambient conditions such as temperature and pressurerequirements at normal or temporarily abnormal exposures.

For holding the membrane (not shown in FIG. 5) in operative position atthe sensor face 511, use of a membrane holding means of the typedisclosed in the above mentioned U.S. patent application Ser. No.164,291, filed June 30, 1980, is preferred for many probe uses; thedisclosure of the just mentioned U.S. patent application is incorporatedherein by way of reference.

Briefly, such holding means include a substantially non-resilient andcreep-resistant ring (not shown in FIG. 5) slidingly fitting onto themembrane-supporting end of jacket 591. A recessed cylindrical endportion (not shown in FIG. 5) of the upper end of jacket 591 may beprovided to define the membrane holding position of the ring and aresilient member, e.g. an O-ring, may be provided between the endportion of jacket 591 and the contacting membrane portion.

Further, sealing members, such as O-rings (not shown in FIG. 5), may beprovided within the probe body in positions where additional sealingconnections are desired, e.g. between mutually contacting ends ofelectrically insulating and electrically conductive probe members. LineS in FIG. 5 is intended to indicate an external sealing connection withthe wall of a fermenter, tube, tank or the like into which the firstprobe portion or exposed end 51 of the sensor protrudes.

Preferably, at least one temperature sensor, e.g. a thermistor, isarranged within probe 5 (not shown in the drawings) and will have leadsfor connection with an instrument (not shown). Two, three or moretemperature sensors can be arranged within the sensor, e.g. within gap581, and in a thermally conducting connection with the ambient mediumcontacting the first probe portion or exposed sensor portion 51.

Preferably, at least one such temperature sensor is used to triggerpassage of heat transfer fluid through the probe when the ambienttemperature rises above a predetermined temperature, e.g. the upper orlower end of the operative temperature range as explained above. Thesame temperature sensor or an additional temperature sensor may be usedto control the rate of passing heat exchange fluid through the probe inresponse to variations of the ambient temperature.

Additional annular tubular air gaps 582, 583 may be provided foradditional thermal insulation if desired and distancing or closing means543, 543a may be inserted for gap definition.

FIG. 6 is a simplified, partially sectioned perspective view of thethermal shield 6 formed by the interfitting pair of tubular members 571,572 of FIG. 5 with channels 578, 579 at their interface. Thermal shield6 is shown to contact the anode connecting tube 555 but this is notbelieved to be critical.

The separate fluid-feeding ends of channels 578, 579 are defined bybores 678, 679 in metal tube 572 while communication of the opposite(i.e. near sensor face) ends is established by annular channel 577extending around the body of anode 55. Thus, heat exchange of the heattransfer fluid in channel 577 through the thermally conducting metal ofthe peripheral anode with the electrolyte in space 56 is maintained anda thermal shield extending through the probe in all its portions whereundesirable temperature impact should be prevented is provided withinthe probe body without impairing sterilization at the outer probesurface because of the thermal insulation between the shield and thejacket as explained above.

EXAMPLE

The following non-limiting example of working the method according tothe invention is given for illustration of thermal MEAC protectionduring heat sterilization.

The electrolyte space of a sensor similar in its essential features tothat illustrated in FIG. 5 was filled with an alkaline potassiumchloride electrolyte and enclosed by a 0.1 mm thickpolytetrafluoroethylene membrane. Its sensitivity towards oxygendissolved in water was determined to be 0.485 microamperes per ppmdissolved oxygen by exposing it to flowing air-saturated waterthermostated at 25° C.

It was then sealed into a pressure vessel together with sufficient waterto maintain a water vapor saturated atmosphere in the vessel at 120° C.The sensing face 511 and part of the cylindrical probe body adjoiningthis face were enclosed within the pressure vessel and the cooling fluidinlet and outlet bores 593 and 594 were accessible outside the vessel.Tap water was passed through the cooling channels to act as heatexchange medium at the rate of 15 liters per hour.

The pressure vessel was heated during 2 hours on an electrically heatedhot plate at a temperature of 120° C. The total pressure in the vesselat this temperature was 4 bars, of which 2 bars were due to the partialpressure of water vapor. A temperature rise of only 2° C. was measuredbetween the inlet and outlet of the stream of cooling water. After thistreatment the pressure vessel was suddenly opened to permit escape ofthe steam and cooled rapidly under fast running water thereby subjectingthe sensor to extremely abrupt changes of ambient pressure andtemperature. This treatment is believed to be the harshest that thesensor might ever be expected to withstand in the course of heatsterilization and would be sufficient to blow the membrane off a sensorwhich did not benefit from the protection of the heat compensationfeature.

No visible effects on the sensor were observable as a consequence of thedescribed sterilization treatment. The sensitivity of the sensor wasredetermined in the same fashion as prior to sterilization. Asensitivity of 0.480 microamperes per ppm was found revealing a downwarddrift of only 1.1% as a result of the sterilization.

The sensor was further shown to be capable of withstanding a series ofrepeated treatments of the same type without harm.

It will be appreciated that the provision of fluid channels extendingthrough an essential portion of a probe body by means of a pair ofinterfitting tubular members with a grooved interface as disclosedherein can be of use for purposes other than thermal protection, e.g.for circulation or regeneration of electrolyte in special applicationsof MEAC-type probes.

Further, while thermal protection of MEAC-type probes is of particularimportance for in-line use of probes in an apparatus that requiresperiodical heat-sterilization, other types of probe operation thatinclude exposure of the probe sensor face to temperatures above or belowthe operative temperature range can be envisaged so that heat protectionof MEACs by the inventive method and a heat-sterilizable probe arepreferred but non-limiting examples of the utility of the presentinvention.

The advantages of the present invention, as well as certain changes andmodifications of the disclosed embodiments thereof, will be readilyapparent to those skilled in the art. It is the applicants' intention tocover by their claims all those changes and modifications which could bemade to the embodiments of the invention herein chosen for the purposeof the disclosure without departing from the spirit and scope of theinvention.

Protection by Letters Patent of this invention in all its aspects as thesame are set forth in the appended claims is sought to the broadestextent that the prior art allows.

What we claim is:
 1. In a membrane-enclosed amperometric probecomprising an elongated and generally cylindrical body having a sensorface at one end of said body; an electrolyte space covered by saidmembrane and in contact with at least two electrodes; an electricalconnecting means extending from said electrodes through said body to aregion near the other end of said body; the improvement which comprisessaid body comprising at least two elongated fluid-passing conduits notbeing in fluid communication with said electrolyte space and extendingin axial direction through said body from a first region near said oneend of said body to a second region near said other end of said body;said conduits having mutually communicating ends in said first regionand separate fluid-feeding ends in said second region.
 2. In amembrane-enclosed amperometric probe comprising an elongated andgenerally cylindrical body having a sensor face at one end of said body;an electrolyte space covered by said membrane and in contact with atleast two electrodes; and electrical connecting means extending fromsaid electrodes through said body to a region near the other end of saidbody; the improvement which comprises said body comprising at least twoelongated fluid-passing conduits extending in axial direction throughsaid body from a first region near said one end of said body to a secondregion near said other end of said body; said conduits having mutuallycommunicating ends in said first region and separate fluid-feeding endsin said second region; and said conduits being in heat-conductiveconnection but not in fluid communication with said electrolyte space attheir mutually communicating ends in said first region near said one endof said body.
 3. In a membrane-enclosed amperometric probe comprising anelongated and generally cylindrical body having a sensor face at one endof said body; an electrolyte space covered by said membrane and incontact with at least two electrodes; an electrical connecting meansextending from said electrodes through said body to a region near theother end of said body; the improvement which comprises said bodycomprising at least two elongated fluid-passing conduits not being influid communication with said electrolyte space and extending in axialdirection through said body from a first region near said one end ofsaid body to a second region near said other end of said body; saidconduits having mutually communicating ends in said first region andseparate fluid-feeding ends in said second region; and said mutuallycommunicating ends being in heat-exchange contact with a heat conductingcell member which, in turn, is in heat exchange contact with saidelectrolyte space.
 4. In a membrane-enclosed amperometric probecomprising an elongated and generally cylindrical body having a sensorface at one end of said body; an electrolyte space covered by saidmembrane and in contact with at least two electrodes; and electricalconnecting means extending from said electrodes through said body to aregion near the other end of said body; the improvement which comprisessaid body comprising at least two elongated fluid-passing conduits notbeing in fluid communication with said electrolyte space and extendingin axial direction through said body from a first region near said oneend of said body to a second region near said other end of said body;said conduits having mutually communicating ends in said first regionand separate fluid-feeding ends in said second region; and said mutuallycommunicating ends opening into a substantially annular confined channelsealed from said electrolyte space but maintained in heat-exchangecontact therewith by a heat-conducting cell member.